Dispersive infrared photothermal beam deflection spectroscopy

A single-beam dispersive photothermal beam deflection (PBD) spectrometer system usable in the IR range is described. A Nernst glower along with a severely modified Perkin-Elmer Model 421 monochromator were used to illuminate a solid sample with modulated monochromatic IR radiation. The resulting photothermal effect generated at the sample's surface caused modulated changes in the refractive index of the air over the sample, causing a probe laser beam grazing the surface to be deflected. The deflections were measured with a bi-cell silicon photodetector. Experiments and analysis of the deflection system showed that lenses used to focus the probe beam at the sample were not needed, and that an improvement in the PBD signal could be obtained by increasing the sample-to-detector distance, but only until a certain sample-detector separation was attained. The dispersive PBD instrument is less sensitive than one based on an interferometer but can be profitably used for studies requiring control of the modulation frequency. Useful spectra of solids were recorded over the 4000-550 cm⁻¹ region using IR excitation in the approximately 5 × 10⁻⁶−5 × 10⁻⁷W range.